CX3CR1 and rheumatoid arthritis: RA FLSs are highly expressed by chemokine C-X3-C motif ligand (CX3CL1) (fractalkine); furthermore, its C-X3-C motif chemokine receptor 1 (CX3CR1) expression is upregulated in the CD4+ and CD8+ T cells of RA patients, suggesting a role for CX3CL1/CX3CR1 in the pathogenesis of RA.